Influence of expulsion and heat extraction resulting from changes to electrode force on liquid metal embrittlement during resistance spot welding

Zinc coatings are generally utilized for manufacturing corrosion-resistant advanced high-strength steels (AHSS). However, in new third generation AHSS (3G-AHSS), zinc from the coating may interact with the steel substrate leading to liquid metal embrittlement (LME) cracking during resistance spot welding (RSW). A critical RSW parameter that influences the LME response of the utilized 3G-AHSS is the electrode force. This study showed that the influence of electrode force on LME depended on whether or not welds experienced expulsion. When welding with low heat input, without expulsion, LME cracking severity decreased as electrode force increased. In such cases, increased force aided with heat extraction during welding, relieving the critical stresses required by LME cracking. In contrast, when welding with high heat input, resulting in expulsion, increased force elevated LME cracking. It was shown that high force increased the sudden indentation of the electrode into the substrate (electrode collapse), leading to rapid cooling of the weld shoulder. The rapid cooling increased the thermal stresses associated with the collapse event, promoting LME. This study established that the electrode force has two distinct roles on LME. When welding below the expulsion current, high force decreased LME. On the other hand, when welding above the expulsion current, more severe LME cracking was observed at high electrode force. The results from this study show that expulsion itself (excluding its association with increased heat input) is a factor contributing to LME cracking, which highlights the importance of considering the expulsion phenomenon in designing LME resistant welding schedules.